Many federal and state regulatory schemes require controlling storm water run-off and water quality, such as levels of pollutants on new developments of land. Before land development, an area will likely have included a variety of natural land features, such as sand dunes, grassy hills and wetlands. The natural land features absorb rainwater and infiltrate storm water runoff into the soil to replenish groundwater and streams. Following land development, however, the area might contain impervious surfaces such as buildings, streets, and parking lots that cover the ground and prevent rainfall infiltration. As a result, storm water runoff can accumulate pollutants such as oil and debris, which then flows into a sewer system or other receiving water bodies.
Bioretention systems for managing and filtering storm water runoff are a well-known Low Impact Design (“LID”) approach to mitigate the impacts of impervious surfaces and manage the flow of storm water runoff on developed land. Bioretention systems utilize soils and both woody and herbaceous plants to remove pollutants, including ultra-fine and dissolved pollutants, from storm water runoff close to their source. The systems mimic the natural (i.e., pre-development) storm water flow from the land. One type of bioretention system includes a tree box filter, which is also referred to as a tree box planter.
In a conventional tree box filter, storm water runoff flows into an in-ground or above ground vault-shaped container with bioretention media, including mulch, and engineered soil. As the tree box filter infiltrates and temporarily stores runoff water, the bioretention media captures particulate matter, including ultra-fine and dissolved pollutants, and allows the treated storm water to percolate through the system. The storm water eventually exists through an outlet in the container into a drainage system or water retention/storage system.
One of the concerns that has emerged is the ability of bioretention systems, including conventional tree box filters, to process large quantities of fluid during peak flow periods without having backups that result in localized flooding of the surrounding areas. Most bioretention systems will have an upper limit for the amount of water that can be filtered at any time, as well as a maximum capacity for the amount of water that can be passed through the system in any event.
To address storm water flow during periods of peak flow and increase the upper limit for fluid flow, some bioretention systems employ an external high-flow bypass mechanism. The feature allows excess fluids to proceed through the drainage system without being filtered during periods of high fluid flow. This conventional high flow bypass is a separate structure, often a separate catch basin or similar device connected to the tree box filter by an external pipe or other mechanism and located downstream from the system. However, because the high flow bypass is an external structure—externally added to the tree box or other bioretention system components—its incorporation with tree box filters requires additional space (to accommodate the external bypass structure), as well as additional design, manufacturing, installation, and maintenance costs.
Another concern is the ability of the bioretention system to remove gross pollutants from incoming storm water prior to releasing it. Ideally, the bioretention system should pre-treat (e.g., using filtration systems) water flow from the developed land prior to releasing it. The entrance of gross pollutants, such as trash, debris, floatables, and coarse sediments, are known to “clog” the system and thus reduce the efficiency. It also increases the maintenance frequency of typical bioretention systems. Pre-treatment apparatus that can remove gross pollutants from the treated flows should be incorporated into the bioretention system in order to minimize land usage. The pretreatment apparatus also should be accessible for intermittent cleaning, repair, and/or other maintenance.
In addition, bioretention systems typically are installed under large concrete or asphalt surfaces to treat storm water that has run over impervious surfaces in commercial, residential, and industrial areas such as median strips, parking lots, sidewalks, and swales. They must be capable of bearing highly variable weight loads. It is desirable for the systems to maximize water storage while occupying as small a “footprint” as possible to minimize land usage and site excavation costs. It is also desirable for the systems to be flexible and accommodate practical needs, such as a need for utility structures and landscape features, of a given landscape area.
Accordingly, what is desired is a bioretention system solving many or all of the foregoing problems, including a bioretention system that can effectively process increased amounts of storm water runoff during peak periods of high fluid flow and can efficiently utilize space within a developed land site. It is another objective of the invention to provide a flexible and economical design that simplifies the design of construction of storm water drainage systems in a landscape area. It is yet another objective of the invention to provide a bioretention system that has fewer and more manageable parts that are relatively easy to maintain and service. It is yet another objective of the invention to provide a bioretention system that has pre-filtration capabilities to remove gross pollutants from storm water runoff before it is released.